1. Field of the Invention
This invention relates to a process for the production of light-colored alkyl oligoglycoside pastes, in which glycoses and fatty alcohols are reacted at elevated temperature in the presence of an acidic catalyst, the water of reaction is continuously removed, the reaction product is neutralized, unreacted fatty alcohol is separated by distillation and the residue is made into a paste with water.
2. Statement of the Related Art
Surface-active alkyl oligoglycosides have long been known as raw materials for the production of detergents. They are normally produced by acid-catalyzed acetalization of sugars (glycoses), more particularly glucose, with fatty alcohols. To obtain high yields, the alcohol component is used in a considerable excess, the water of reaction released is continuously removed from the reaction equilibrium and the reaction is terminated when virtually all the glucose, i.e. at least 99% by weight, has reacted off. The acidic catalyst is then neutralized and the excess fatty alcohol is separated by distillation. U.S. Pat. Nos. 3,839,318, 3,450,690, EP 0 132 046 A1 and WO 90/03977 are cited as representative of the large number of prior-art processes.
A serious technical problem in the production of alkyl oligoglycosides lies in the fact that, after separation of the fatty alcohol, the reaction products show strong discoloration so that, for aesthetic reasons, they can only be used as detergent raw materials after preliminary bleaching.
According to EP 0 077 167, the color quality of surface-active alkyl oligoglycosides can be improved by using a typical acidic catalyst together with an acidic reducing agent from the group consisting of phosphorous acid, hypophosphorous acid, sulfurous acid, hyposulfurous acid, nitrous acid and/or hyponitrous acid or the corresponding salts in the production of the alkyl oligoglycosides.
According to the teaching of EP 0 102 558, light-colored C.sub.3-5 alkyl glucosides are obtained by carrying out the production of the alkyl glucosides in the presence of an acidic catalyst and an at least equivalent quantity of an alkali metal salt of a boric acid, preferably sodium perborate.
Finally, according to EP 0 165 721, color-stable products can be obtained by treating an aqueous solution of a surface-active alkyl oligoglucoside first with an oxidizing agent, preferably a hydrogen peroxide solution, and then with a sulfur dioxide source, for example an aqueous solution of sodium bisulfite.
In addition, it is known from the prior art that, after separation of the fatty alcohol, light-colored alkyl oligoglycosides can be obtained if neutralization of the acidic catalyst is carried out with magnesium oxide used in an excess of 200 to 500 mol-% and preferably 400 mol-%, based on the hydrogen ions present in the reaction mixture. However, the difficulty here is that magnesium oxide is difficult to incorporate in the reaction mixture. This is a major disadvantage, particularly for continuous operation, because the neutralization with magnesium oxide involves an additional dispersion step.
Another way of avoiding the problem of dispersing magnesium oxide in the reaction mixture is to use an aqueous sodium hydroxide solution, optionally with addition of MgO, as the neutralization base. However, the use of alkali metal hydroxides leads to discolored products both where stoichiometric quantities and where overstoichio-metric quantities are used. In addition, serious foaming problems occur during distillation, necessitating additional technical measures and adding to the overall costs of the process. In addition, the resulting products can only be bleached with considerable difficulty, if at all, by conventional methods.
Finally, another problem affecting the economic production of alkyl oligoglycosides lies in the fact that the fatty alcohol accumulating during the distillation phase cannot readily be returned to the acetalization step. In view of a large number of impurities, the fatty alcohol has to be subjected instead to a reducing treatment with borohydrides because otherwise the alkyl oligoglycosides would be adversely affected both in regard to their color quality and in regard to their performance properties.
Accordingly, the problem addressed by the present invention was to provide an improved process for the production of light-colored alkyl oligoglycoside pastes which would be free from the above-mentioned disadvantages.